Heat-sensitive copy-sheet



United States Patent 3,394,042 HEAT-SENSITIVE COPY-SHEET Louis Eugene Wingert, White Bear Lake, and Victor R.

lFrauer, Roseville, Minm, assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Sept. 13, 1963, Ser. No. 308,839 3 Claims. (Cl. 161-6) This invention relates to heat-sensitive copy-sheets which are useful in copying differentially radiation-absorptive graphic originals by the thermographic frontprint process involving brief exposure of the printed original to intense infra-red radiation applied through the contacting copy-sheet.

Copy-sheets of sufiicient thinness to be adequately transmissive of radiation required in the front-printing process have in the past been found to possess certain disadvantages. They are subject to curling when heated or handled. Underlying originals or copies may be seen through the semi-transparent copy-sheet. The sheet is fragile and may be wrinkled or torn on handling, as in collating, folding, or filing. However these sheets produce clear and sharp copies by an extremely simple, rapid and economical copying process and have found wide acceptance in the office copying and allied fields.

Typically, these prior art copy-sheets consist of a waxy or resinous visibly heat-sensitive coating on a thin and semi-transparent paper-like carrier through which the image is to be viewed. The coating may contain fusible waxes or resins in light-diffusing heat-transparentizable form, although coatings of chemically inter-reactive components including a fusible waxy reactant have been generally preferred. As examples may be cited mixtures of ferric stearate and a phenol, or of silver behenate and a hydroxyaromatic reducing agent. A minor amount of a film-forming binder is usually included. These coatings may additionally be covered by a further thin coating of polymeric film-forming material, usually containing opacifying or colored pigments, for protecting the sensitive coatings from abrasion and for increasing the visible contrast between image and background areas.

The requirement of close heat-conductive contact between the sensitive coating and the printed original during the copying process has effectively restricted the thickness of the protective surface coating so that only a moderate opacifying effect could be obtained by such means. Similarly the necessity for transmission of infra-red radiation has made necessary the use of very thin paper-like carriers for the reactive heat-sensitive coatings and the overlying protective opacif ying layers.

The present invention makes available a copy-sheet structure which employs the same image-formation mechanism and permits the same rapid and accurate front print thermographic copying of printed originals, while at the same time providing the handling characteristics of much heavier Weight papers than have hitherto been thought possible for such applications, together with a gratifying increase in resistance to curling and in image contrast.

In accordance with the invention, it is now found possible to substitute for the previously employed protective surface coating an adherently bonded thin light-diffusing paper-like fibrous web. A degree of opacity at least equal to, and in most cases much greater than that obtained with the previously known pigmented protective coating is obtained. The fibrous structure provides a highly desirable reinforcing action, markedly improving the resistance of the sheet to folding, handling, tearing and curling. The receptivity of the sheet to markings applied with ink or pencil is improved.

3,394,042 Patented July 23, 1968 The structure is illustrated in schematic cross-section in FIGURE 1, and a method of manufacture is similarly illustrated in FIGURE 2. The copy-sheet 40 includes a light-diffusing fibrous paper-like backing layer, a transparent fibrous reinforcing face layer, and an intermediate, adherently bonded, visibly heat-sensitive, adhesive-andfiller layer. In the method shown in FIGURE 2, the transparent face sheet 30 from supply roll 29 is drawn around guide rollers 31 and between pressure roller 33 and applicator roller 32, the latter being supplied with adhesive solution from trough 34. The lightly uniformly coated sheet then passes around guide roller 35, through an oven 36 for controlled removal of solvent, and thence to combining rollers 26, 27. The fibrous backing layer is previously supplied, by suitable coating and drying means not shown, with a thin uniform visibly heat-sensitive surface coating, the coated sheet 37 being drawn from supply roll 28 into face-to-face pressure-contact with the adhesive-coated sheet at the combining rollers as shown. The completed heat-sensitive copy-sheet 40 is drawn past guide roller 38 and wound up in the form of storage roll 39.

in one variation the materials of the visibly heatsensitive layer are mixed with the materials of the adhesive layer and the whole is applied as a unitary coating at applicator roller 32, the coated sheet then being combined with the previously untreated paper backing. Separation of the two compositions is preferred as providing maximum concentration of reactants and density of image, although of course requiring two separate coating and drying operations.

In the therrnographic front-printing copy process the heat-sensitive copy-sheet as thus described is placed with the light-diffusing paper surface in contact with the printed original which is then exposed briefly to intense infrared radiation through the copy-sheet. The heat pattern created at the image areas is conducted through the paper layer to the heat-sensitive layer. The resulting image is clearly visible through the adhesive layer and the outer transparent face layer. Thus there is produced a rightreading copy having sharp and clear image areas, having adequate strength and rigidity for easy and safe handling, having good curl resistance, and being receptive of ink and pencil markings on both surfaces.

Specific illustrative examples, in which all proportions are given in parts by weight unless otherwise stated, will now be set forth.

EXAMPLE 1 A coating composition is first supplied by intimately dispersing 19.24 parts of ferric stearate and 7.31 parts of cornstarch in a solution of 4.70 parts of ethyl cellulose in 68.4 parts acetone, by prolonged milling in a ball mill, and then adding 83 parts of a solution containing 5.84 parts of ethyl cellulose, 6.56 parts of tertiarybutyl catechol, and 7.85 parts of triphenyl phosphate in 79.5 parts of acetone.

The composition is coated from a bar coater at an orifice of 3 mils (0.002 inch) on a thin pigmented bleached kraft paper (Berkley White kraft, weighing 8 lbs/ream of 20 x 30/500 sheets and having a caliper of about one mil). The coating when dried weighs 0.7 gram per sq. ft. It is visibly heat-sensitive, changing from a light buff to a bluish black when briefly heated at C. The coated surface has a greasy or waxy feel and does not accept ink or pencil markings. The sheet curls badly on exposure to changes in humidity.

Separately a transparent highly calendered thin glassine paper (Insulglass glassine weighing 20 lb./ ream of 24 x 36/500 sheets, calipering about one mil) is coated to a dry coating weight of 0.8 gram per sq. ft. with a composition containing: 18 parts of hard inert thermoplastic styrene homolog copolymer softening at about 120 C. (Piccotex 120); 20 parts of chlorinated hydrocarbon resin having a specific gravity within the range of 1.618 to 1.629 and distilling between 385 and 420 C. (Aroclor 1260); 7 parts of soft alpha-methylstyrene resin having a viscosity of 700 to 1000 centipoise at 60 C. and a boiling range of 150300 C. at mm. Hg (276-V9 methylstyrene resin); 3.75 parts of polyvinyl ethyl ether (from 25% solution in hexane, the solution having a specific gravity of 0.780, such as EHBM resin solution); and 40 parts of commercial heptane. Drying is terminated when the coating is firm and non-flowing but still strongly tacky to the touch, at which point the coated surface is pressed into firm smooth contact with the heat-sensitive coating of the first-described coated paper. The composite sheet is allowed to stand under normal room conditions for an additional 48 hours during which time a slight increase in rigidity occurs, presumably either by loss of final traces of solvent or by re-distribution of plasticizer. It has a caliper of 2.5 mils.

The copy-sheet product produces sharp clear copies of graphic originals by front-printing in the thermographic process. The image has good contrast against the pale buff background and is clearly and distinctly visible through the glassine paper and adhesive. The sheet may be folded and otherwise handled without tearing or breaking, and is adequately rigid to be inserted into file folders and otherwise handled with the same facility afforded by the usual bond papers employed for office correspondence, records, and similar documents.

EXAMPLE 2 The kraft paper of Example 1 is coated at 0.4 gram/sq. ft. dry weight with a com-position containing parts of intimately dispersed silver behenate, 2 parts of phthalazinone resin, 2 parts of behenoyl pyrogallol, 10 parts of HE-40 oily hydrocarbon plasticizer, 0.002 part of Alizarine Irisol N blue dye, 0.3 part of tetrachlorophthalic anhydride stabilizer, and 172 parts of acetone. The dried coated sheet is combined under pressure with the adhesive-coated glassine sheet of Example 1 in the manner there described. The completed product is an effective heat-sensitive copy-sheet for front-printing, producing clearly visible dense black images on a white background. The sheet calipers 2.5 mils and has adequate strength and rigidly to effectively withstand all normal office handling.

The sucrose acetate isobutyrate resin is a tacky material having a molecular weight of about 850 and a viscosity of about 100,000 centipoises at 30 C. and about 4800 cps. at 50 C.

EXAMPLE 3 A composition containing 6.6 parts by weight of nickel stearate, 4.6 parts of polyvinyl acetate resin, 2.3 parts of Syloid 244 silica powder, and 86.5 parts of acetone is prepared in the ball mill and is coated on this pigmented bleached kraft paper (Ecusta Versatile paper, having a caliper of 1.5 mils and a basis weight of 20 lbs/ream of 24 x 36/500 sheets) at a coating orifice of two mils. The sheet is dried. Separately a composition is prepared containing 0.2 part by weight of N,N-dibenzyldithiooxamide, 20 parts of Piccotex 120 resin, 18 parts of Aroclor 1260 resin, 7 parts of sucrose acetate isobutyrate resin, and parts of EHBM resin solution in 40 parts of heptane, and this composition is coated on -11). glassine as used in Example 1 at a coating orifice of three mils and dried to the non-flowing tacky state. The two coated sheets are pressed together in face-to-face contact and the resulting rather limp compound sheet is permitted to stand under normal room conditions for 48 hours. The product has a weight of 9.6 grams per sq. ft. and a caliper of 2.7 mils. It produces clear and sharp copies of printed originals by front-printing in the thermographic process, and the copies are resistant to folding and handling as required for ofiice records and the like.

Stiffness value, for example as measured on test machines such as the Taber Stiffness Tester, is a measure of the handling properties of paper-like sheet materials. Typical papers as used for office correspondence, such as bond papers of 16 to 20 lb. basis weight (17 x 22/500), show a stiffness value in the machine direction, as determined on the Taber instrucent, of about 1.3-1.4 gm. cm. whereas representative prior art front-print heat-sensitive copy-sheets when similarly tested have in most instances shown stiffness values of less than about 0.5 gm. cm. These values indicate the bending moment required to flex a test specimen through an angle of 15 degrees, the specimen being 1 /2 inch in width. The sheets of Examples 2 and 3, on the other hand, while being equally effective as copy-sheets in providing clear sharp contrasty copies by front-printing techniques, are fully equivalent in stifiness value to the ofiice bond papers, showing values of 1.3 to 1.4 gm. cm. on the same machine.

EXAMPLE 4 The image-forming reactants and adhesive bonding components may be incorporated in a single coating composition and applied in a single coating step. A typical composition for this purpose contains 10 parts by weight of silver behenate, 2 parts of phthalazinone, 2 parts of behenoyl pyrogallol, 0.3 part of tetrachlorophthalic anhydride, 0.002 part of blue dye, 2.5 parts of sucrose acetate isobutyrate resin, 16.4 parts of Piccotex 120 resin, 15 parts of EHBM resin solution, and 16.3 parts of EHBC resin solution, in parts of acetone and 25 ployed in the EHBM solution. The composition is 25 solution in hexane of a polyvinyl ethyl ether having a lower molecular weight than the similar polymer employed in the iEHBM solution. The composition is applied in a thin uniform coating to the 20-lb. glassine at an orifice of 1.5 mils, the coating is dried at moderately elevated temperature until firm and tacky, and the coated surface is then pressed into contact with the uncoated surface of 8-lb. Schweitzer kraft paper. After further aging the sheet is found to have a Taber stiffness value of 1.3 gm. cm., a caliper of 2.5 mils and a weight of 5.4 grams/sq. ft., to produce clear sharp copies of printed originals by the front-printing process, and to have handling properties essentially equivalent to those of bond papers as used in office correspondence.

EXAMPLE 5 Where the previous examples have described chemically reactive image-forming components, the copy-sheet of the present example operates through a different mechanism. The 20-lb. glassine of previous examples is first coated with a mixture of 10 parts by weight of 50% polystyrene latex and 3 parts of 50% polymethylmethacrylate latex, applied through a 1.5 mil orifice and dried to a white lightdiffusing surface which becomes transparent when heated, for example to 150 C. Separately, the 8 lb. Schweitzer kraft paper is coated with the same thickness of a composition containing 20 parts of Piccotex resin, 18 parts of Aroclor 1260 resin, 7 parts of sucrose acetate isobutyrate resin, 15 parts of EHBM resin solution, 0.1 part of Alizarine Irisol N blue dye, and 40 parts of heptane, and the coating is dried until firm but still tacky. The two sheets are pressed together in face-toface relationship and permitted to age for 48 hours. The product is useful in making copies by front-printing, the copy appearing as a blue image against a white background. The sheet has good handling properties similar to those of ofiice bond paper.

The specific examples given hereinabove have each provided a copy-sheet on which the image produced by r the thermographic process is viewed against a contrast;

ing opaque or opalescent background obtained, at least in part, from the light-diffusing fibrous back sheet. The invention is not to be restricted to such structure, but encompasses heat-sensitive copy-sheets in which the visibly heat-sensitive composition or layer is itself transparent and is incorporated between transparent face and back sheets, for example between two layers of glassine or two transparent films of materials such as cellulose ethers or esters, Mylar or other polyesters, or polyvinyl resins. The structure permits the preparation of copies in the form of opaque images on a transparent background, using for example the reactant composition of Example 2 or 4 in a transparent adhesive composition, or in the form of transparent images on an opaque background using the transparentizable coating of Example 5 in the absence of the dye. The resulting transparencies are particularly useful for image projection purposes, as by means of the overhead projectors used in lecture rooms, since they are easily and rapidly prepared from printed originals by the convenient thermographic process.

These multi-layer copy-sheets have the further advantage that the reactant materials are completely enclosed or encased between the surface webs and thereby are protected from external influences while at the same time being kept from contact with other copysheets or materials. Furthermore, the sheet retains the desirable marking properties of Office papers, namely of being fully receptive of pencil or ink markings on either surface. These advantages are useful in copy-sheet structures designed for copying processes other than thermographic front-printing or back-printing, as will be illustrated in the following further example.

EXAMPLE 6 Thin paper as used in the previous examples is first pre-sized with a minimum uniform application of a polyisobutylene resin (Vistanex L-l40) and is then coated with a solution containing 6.2 parts by weight of ethyl cellulose, 0.015 part of Erythrosin B dye, 0.131 part of 4-methoxy-1-hydroxy-naphthalene, 6 parts of butyl alcohol and 88 parts of acetone. The sheet is coated and dried in the absence of actinic radiation.

A transparent glassine face sheet is separately coated with a mixture of 25.15 parts by weight of zinc oxide, 18.97 parts of silver behenate, 5.03 parts of phthalazinone, 30 parts of polyvinyl acetate, 5 parts of powdered thermoplastic terpene resin (Piccolyte Sl35), 0.33 part of tetrachlorophthalic anhydride, 32.60 parts of ditertiarybutyl-p-cresol, and 370 parts of acetone. Over the dried coating is deposited a further thin layer of Vistanex Ll40 polyisobutylene, and the sheet is pressed into smooth .face-to-face contact with the coated paper previously prepared, this last operation also being conducted essentially in the absence of actinic radiation.

The completed copy-sheet is just under three mils in caliper and has essentially the handling characteristics of office bond papers.

The sheet is first exposed to a light-image, for example from a projector or by contact printing through a transparency; but exposure may equally well be carried out with the sheet in front-print or reflex position. The actinic radiation causes conversion of the methoxyhydroxynaphthalene to a form in which it no longer reacts with the silver behenate on briefly heating the sheet; and exposure is continued until essentially complete conversion is achieved at the light-struck background areas. The sheet is then briefly heated, e.g., for a few seconds at 110 C. A dense black image is formed, corresponding to the opaque or light-absorptive image areas of the original.

It will be apparent from the foregoing that the visibly heat-sensitive layer may 'be widely varied both as to specific components and as to their arrangement within the structure. Thus the chemical reactants, Where employed, may be present throughout a single stratum or in separate strata and in either order with respect to the transparent face. The fluid coating compositions may be applied to but one of the surface Webs or to both, although preferably at least to the transparent face web, i.e., the glassine in the examples, since the apparent transparency of that sheet is appreciably improved there by. Similarly, bonding of the dried adhesive coating to the light-diffusing paper back Web, as in Example 4, provides maximum light diffusion at the paper surface and maximum contrast with the darkened image areas.

The adhesives employed must be sufficiently flexible to withstand folding and crumpling of the completed sheet without permitting separation of the surface layers, but not so soft that they do not add significantly to the stiffness value of the product. Despite the waxy nature of reactant components such for example as the metal soaps, the adhesives are found to adhere well to the coatings containing such components as well as to the paper surface. Like the other materials, the adhesives must transmit rather than absorb the infra-red radiation. They must be reasonably heat-resistant, must not flow under prolonged moderate pressure, and must be resistant to deterioration on long aging. The components employed in the specific examples are selected and proportioned with these requirements in mind; and other adhesive formulations are known or may be devised from the same or other combinations of hard and soft resins and polymers, plasticizers, pigments, fillers, stabilizers and preservatives, and other well-recognized component materials by those skilled in the adhesive art in the light of the requirements listed, the specific examples here provided being illustrative rather than limitative.

What is claimed is as follows:

1. A heat-sensitive copy-sheet useful in the copying of graphic originals by the thermographic front-printing process, having a Taber stiffness value of about 1.3 gm. cm., and having handling properties essentially equivalent to those of ofiice bond papers under conditions applicable to the normal handling of office correspondence, said copysheet comprising a thin transparent glassine face sheet, a light-diifusing fibrous paper-thin back sheet, and an intermediate layer bonding said sheets together in close uniform congruency and comprising chemically reactive components for providing a permanent visible change by chemical reaction on said layer being momentarily heated to a conversion temperature not higher than about C., and a flexible firm cohesive long-aging water-soluble resinous adhesive.

2. A heat-sensitive copy-sheet useful in the copying of graphic originals by the thermograpliic front printing process, having a Taber stiffness value of about 1.3 gm. cm., and having handling properties essentially equivalent to those of oflice bond papers under conditions applicable to the normal handling of oflice correspondence, said copy-sheet comprising, in order a thin transparent glassine, a flexible firm cohesive long-aging water-insoluble resinous adhesive layer, a visibly heat-sensitive layer comprising chemically reactive components for providing a permanent visible change by chemical reaction on said layer being momentarily heated to a conversion temperature not higher than about 150 C., and a fibrous lightdiffusing paper-thin back sheet.

3. A heat-sensitive copy-sheet having essentially the white opaque appearance and the stiffness and handling characteristics of office bond papers and comprising, in order: a transparent dense glassine face sheet about one mil in thickness; a thin transparent firm flexible cohesive long-aging water-insoluble resinous adhesive layer including a hard resin and a soft tacky resin; a visibly heatsensitive layer including a silver soap, a silver reducing agent, a silver image toner, and a resinous binder; and a light-diffusing paper back sheet about one mil in thickness; the total thickness of the unitary copy-sheet being about 2 /2 mils; and said copy-sheet being transmissive of infra-red radiation.

(References on following page) References Cited UNITED STATES PATENTS Clark et a1.

Owen.

Menzer.

Smith et a1 161-229 Adhikary 1616 Schulz et a1. 161-269 X Vander Weel 11736.7 Wartman 117-362 ROBERT F. BURNETT, Primary Examiner.

W. A. POWELL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 3,594 ,042 July 23 1968 Louis Eugene Wingert et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 60, ".003" should readav- .002 Column 3, line 37, after "phthalazinone" insert silver image toner, 2.5 parts of sucrose acetate isobutyrate line 51, "centipoises should read centipoise line 58, "this" should read thin Column 4, line 9, "instrucent" should read instrument line 33, after "25" insert parts of heptane. The "EHBC" solution is a low-viscosity Column 6, line 40, "paper-thin" should read thin paper line 47, "soluble should read insoluble line 54, after "in order" insert a comma; line 61,

"paperthin" should read thin paper (SEAL) Signed and sealed this 16th day of December 1969 Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A HEAT-SENSITIVE COPY-SHEET USEFUL IN THE COPYING OF GRAPHIC ORIGINALS BY THE THERMOGRAPHIC FRONT-PRINTING PROCESS, HAVING A TABER STIFFNESS VALUE OF ABOUT 1.3 GM. CM., AND HAVING HANDLING PROPERTIES ESSENTIALLY EQUIVALENT TO THOSE OF OFFICE BOND PAPERS UNDER CONDITIONS APPLICABLE TO THE NORMAL HANDLING OF OFFICE CORRESPONDENCE, SAID COPYSHEET COMPRISING A THIN TRANSPARENT GLASSINE FACE SHEET, A LIGHT-DIFFUSING FIBROUS PAPER-THIN BACK SHEET, AND AN INTERMEDIATE LAYER BONDING SAID SHEETS TOGETHER IN CLOSE UNIFORM CONGUENCY AND COMPRISING CHEMICALLY REACTIVE 